1. Field of the Invention
The present invention relates to tri-state chopper circuit and its application to frequency conversion, in particular to frequency conversion that suppresses harmonics mixing.
2. Description of the Prior Art
Direct conversion receiver is well known in prior art. FIG. 1 depicts a functional block diagram of a typical direct conversion receiver 100, which comprises: a pre-filter 110; a LNA (low-noise amplifier) 120; an in-phase (I) path comprising a first mixer 130_I, a first LPF (low pass filter) 140_I, and a first ADC (analog-digital converter) 150_I; and a quadrature (Q) path comprising a second mixer 130_Q, a second LPF (low pass filter) 140_Q, and a second ADC (analog-digital converter) 150_Q. Pre-filter 110 performs a preliminary filtering on a RF (radio frequency) input RF_IN and passes an output to LNA 120, which performs a low-noise amplification on the output of pre-filter 110 and generates a RF signal 122 provided as an input to both the in-phase (I) path and the quadrature (Q) path. The in-phase path receives the RF signal 122 and converts it into a first digital baseband signal BB_I by mixing the RF signal 122 with an in-phase clock LO_I using mixer 130_I, filtering an output of mixer 130_I using LPF 140_I, and converting the output of LPF 140_I into the digital baseband signal BB_I using ADC 150_I. The quadrature path receives the RF signal 122 and converts it into a second digital baseband signal BB_Q by mixing the RF signal 122 with a quadrature clock LO_Q using mixer 130_Q, filtering an output of mixer 130_Q using LPF 140_Q, and converting the output of LPF 140_Q into the digital baseband signal BB_Q using ADC 150_Q. In general, the input signal RF_IN is a wide-band signal that contains many spectral components, among which only a narrow-band component is to be selected. To satisfy the condition of “direct conversion,” both the in-phase clock LO_I and the quadrature clock LO_Q must have the same frequency as the desired narrow-band component, and also the phase difference between the in-phase clock LO_I and the quadrature clock LO_Q must be 90 degrees.
Although the principle of direct conversion receiver has been well known in prior art, it is very difficult to apply direct conversion to a TV tuner due to a problem known as “harmonic mixing” caused by an undesired but inevitable mixing within the two mixers (130_I and 130_Q). In particular, a practical mixer is subject to generating spurious mixing products among its input RF signal and odd-order harmonics of the LO clock. For instance, a tuner needs to be tuned to select a channel among a plurality of channels ranging from 47 MHz to 862 MHz. If the tuner were tuned to 100 MHz using direct conversion architecture, both LO clocks (LO_I and LO_Q) must also be 100 MHz in frequency. While the desired channel (at 100 MHz) can be successfully converted into the two baseband signals, the signal of an undesired channel at 300 MHz will also be converted and become a part of the two baseband signals, since the undesired signal at 300 MHz will also be mixed with the 3rd harmonics of the LO signals. A mixer can be implemented either as a “multiplying mixer” or a “switching mixer”; neither is immune from the “harmonic mixing” problem.
What is needed is a method to perform frequency conversion without having spurious mixing problems caused by harmonic mixing.